As a palladium-containing catalyst for producing an α,β-unsaturated carboxylic acid through liquid-phase oxidation of an olefin with molecular oxygen, for example, a palladium metal catalyst obtained by reducing palladium in an oxidation state by an olefin has been proposed in Patent Documents 1 and 2. Further, a palladium metal catalyst obtained by reducing a palladium compound to palladium metal using formalin, hydrazine, hydrogen, methanol, or an olefin such as ethylene, propylene, or butene as a reducing agent is described in Patent Document 3.    Patent Document 1: International Publication No. WO 02/083,299    Patent Document 2: Japanese Patent Application Laid-Open No. Sho 60-155,148    Patent Document 3: U.S. Pat. No. 4,016,200